Hard Nodules on Dorsum of Feet
The most likely diagnoses for hard palpable nodules on the dorsum of the feet include gouty tophi, rheumatoid nodules, and acrodermatitis chronica atrophicans (a late manifestation of Lyme disease), with ultrasound being the first-line imaging modality to characterize these lesions. 1
Differential Diagnoses
Primary Considerations
Gouty tophi are the most common cause of hard nodules over bony prominences of the feet and can be identified by ultrasound due to their characteristic sound shadow. 1
Rheumatoid nodules occur over joints and extensor surfaces in patients with active rheumatoid arthritis, typically in RF-positive patients, and present as firm subcutaneous nodules with palisading granulomatous inflammation on histology. 2
Acrodermatitis chronica atrophicans develops on the extensor surfaces of hands and feet years after Lyme disease infection (0.5–8 years), initially presenting with bluish-red discoloration and doughy swelling, then progressing to nodules over bony prominences such as the elbow or patella. 1
Important Exclusions
Erythema nodosum should be excluded but is unlikely given the location—EN characteristically affects the anterior tibial areas bilaterally and symmetrically, not the dorsum of the feet, and presents as tender, raised, red or violet nodules of 1–5 cm diameter that do not ulcerate. 3, 4, 5
Nodular vasculitis presents with subcutaneous nodules that may ulcerate and are surrounded by livedo racemosa, often associated with tuberculosis infection. 6
Tendinopathy nodules occur along tendon insertions with load-related pain and well-localized tenderness, distinguishable by their relationship to specific tendons. 1, 7
Diagnostic Approach
Clinical Examination
Examine for:
- Joint involvement: Active arthritis, joint deformities, or synovitis suggests rheumatoid nodules 2
- Skin changes: Bluish-red discoloration with skin atrophy ("cigarette paper skin") and prominent veins suggests acrodermatitis chronica atrophicans 1
- Peripheral neuropathy: Sensory loss in the affected extremity accompanies two-thirds of acrodermatitis chronica atrophicans cases 1
- Nodule characteristics: Firm, non-tender nodules over bony prominences favor tophi or rheumatoid nodules 1
First-Line Investigation
Musculoskeletal ultrasound is the initial imaging modality of choice, using high-frequency transducers (10 MHz or higher) with the patient supine for dorsal scans. 1
Standard scanning protocol includes:
- Dorsal longitudinal scans moving from proximal to distal 1
- Dorsal transverse scans 1
- Lateral scans for first and fifth toes 1
Ultrasound can differentiate:
- Tophi: Characteristic sound shadow 1
- Rheumatoid nodules: Solid hypoechoic masses 1
- Tenosynovial disease: Fluid or thickening around tendons 1
Laboratory Testing
Order:
- Serum uric acid if tophi suspected 1
- Rheumatoid factor and anti-CCP antibodies if rheumatoid nodules suspected 2
- Lyme serology (IgG) if acrodermatitis chronica atrophicans suspected, particularly with appropriate epidemiology (endemic area exposure) 1
- Complete blood count to assess for eosinophilia in parasitic infections 7
When to Biopsy
Skin biopsy is indicated when:
- Clinical presentation is atypical 8
- Lesions fail to improve after initial therapy 8
- Diagnosis remains uncertain after ultrasound and laboratory testing 1
Histopathology will show:
- Tophi: Urate crystal deposits 1
- Rheumatoid nodules: Palisading granulomatous inflammation with central necrobiosis 2
- Acrodermatitis chronica atrophicans: Pronounced lymphoplasmacellular infiltration with or without atrophy 1
Management
Gouty Tophi
Treat with urate-lowering therapy to achieve target serum uric acid <6 mg/dL; surgical excision may be considered for large, symptomatic tophi. 1
Rheumatoid Nodules
Optimize disease-modifying antirheumatic drug (DMARD) therapy for underlying rheumatoid arthritis; consider rheumatology consultation for persistent symptomatic nodules. 1, 2
Acrodermatitis Chronica Atrophicans
Treat with intravenous ceftriaxone 2 g once daily for 2–4 weeks, with cefotaxime or penicillin G as alternatives. 1 Response is usually slow and may be incomplete; re-treatment is not recommended unless relapse is documented by objective measures. 1
Common Pitfalls
Do not assume erythema nodosum based solely on "nodules on the feet"—EN has a characteristic anterior tibial distribution and does not typically affect the dorsum of the feet. 3, 4
Do not delay Lyme serology in patients with appropriate epidemiologic exposure and chronic nodules on extensor surfaces, as acrodermatitis chronica atrophicans develops years after initial infection and may lack history of erythema migrans. 1
Do not perform routine biopsy for clinically typical presentations when ultrasound and laboratory testing provide sufficient diagnostic information. 1, 8
Consider diabetic foot complications if the patient has diabetes—probe any overlying ulcers with a sterile blunt metal probe to assess for underlying osteomyelitis, as a positive probe-to-bone test suggests bone involvement. 1